The development and acceptance of wind energy as a clean and productive source of alternative energy is proliferating. Wind energy can be captured by a wind turbine generator, which is a rotating machine that converts the kinetic energy of the wind into mechanical energy, and the mechanical energy subsequently into electrical power. Common horizontal-axis wind turbines include a tower, a nacelle located at the apex of the tower, and a rotor that is supported in the nacelle by means of a shaft. The shaft couples the rotor either directly or indirectly with a rotor assembly of a generator housed inside the nacelle. A plurality of wind turbine generators may be arranged together to form a wind park or wind power plant.
Wind power plants are coupled to an electrical network or a power grid. In some situations, wind power plants are situated in rural areas, and infrastructure is sometimes inadequately set up to support the wind power plant electrical distribution. As a result, such electrical grids which are far from the main generation units are characterized by low fault level at the point of connection, also known as a “weak grid”. A wind power plant connection to a weak grid faces challenges in both steady state and transient conditions.
A weak transmission grid or electrical grid or power grid or network grid is characterized by high grid impedance due to the low fault level. In a weak grid, it is typically necessary to take voltage level and fluctuations into account because there is a probability that the values might exceed the requirements in the standards when load and power production cases are considered. Where a wind energy plant or a wind power plant is in a weak grid environment, or connected or coupled to a weak grid, due to the higher impedance of the grid, the amount of wind energy that can be pushed into the grid at the point of connection is limited because of a relatively large fluctuation of the voltage and due to stability of the grid system. Further, during normal operation, voltages in a weak grid can exceed or go below the normal continuous operating voltage range of the grid.
This can lead to situations, in which voltage fluctuations can result in unstable steady state operation, for example, leading to a re-triggering loop due to voltage instability at the grid side. In addition, the weak grid brings about increased reactive power losses, which can require extra dynamic compensation, this leads to less reactive power being available to boost the grid voltage, leading to larger voltage fluctuations, and resulting in unacceptable voltage oscillations in the grid.
During transient conditions such as system faults, the wind turbines connected to a weak grid face challenges such as troubled recovery from the fault, large oscillations in active power and voltage, and often face unwanted trips due to overvoltage protection settings at the wind turbine or wind power plant level.
There is as such a desire for a method for operating a wind power plant in a weak grid environment which can continue to be connected to and supporting the grid during small voltage disturbances and fault events.